Direction Reversal of D. C. Motor

As we know the torque developed M≈ IaΦ.

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—Thus to develop torque in reverse direction either Armature current should be reversed.

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—Or Φ should be reversed.

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—To reverse Φ means Field Current should be reversed. 

Figure : Armature Reversal

Figure : Field Reversal

  

—In both the conditions because of inductive nature of the windings current and thus torque would not reverse at the instant of voltage reversal. 

◦ Figure : Time Delay

Field winding being more inductive than Armature would take larger time to provide reversal, thus Armature reversal is preferred.

Limitations of reversals

1. —It is clear that even if the voltage polarity of armature is reversed the current and hence torque direction does not change instantaneously. 
2. During this overlap when the torque and hence Eb is opposite to theapplied voltage, there will be practically ∞ current (limited only byarmature resistance) flowing in the armature circuit. 
3. So it becomes essential that before we reverse the polarity of armaturevoltage the earlier torque should cease and Eb should come to zerolevel.
4. Now the first step to achieve this is to make armature voltage zero.  
5. But this will not help because of the fact that motor will continue its rotation because of its inertia and keep generating Eb. 
6. Also the Ia will take its own time depending upon armature circuit timeconstant to come to its zero value after the removal of armaturevoltage.
7. This will take some time and thus instantaneous reversal is not be possible. 
8. To over come this we require the method wherein we forcibly make the Ia / T and Eb zero and proceed with reversing. 
9. This phenomenon is called as 'Braking'. 
10. The normal sequence of operation for reversal, therefore, would be Motoring followed by braking followed by reverse motoring.